Cross-product brands with a land-based presence tend to be up there. As such, you can't look much past brands like bet365, Will Hill, Ladbrokes, Paddy Power in terms of player volume. Player value is altogether different of course.
Interesting piece on an affiliate website that covers the UK - I guess similar information will be available at a national level elsewhere in regulated markets.
UK Online Casino Total Number of Players Active Monthly Players Active Daily Players Online Since
888 Casino
#1 Popular UK Casino
5,240,000 520,000 170,000 1997
Ladbrokes
#2 Popular UK Casino 3,850,000 420,000 190,000 2000
PartyCasino
#3 Popular UK Casino 6,250,000 340,000 125,000 2006
Bet365
#4 Popular UK Casino 2,150,000 300,000 105,000 2001
William Hill
#5 Popular UK Casino 3,250,000 260,000 85,000 1998
JackpotJoy
#6 Popular UK Casino 2,200,000 255,000 82,000 2001
Spin Palace
#7 Popular UK Casino 1,350,000 210,000 52,000 2001
32 Red
#8 Popular UK Casino 1,200,000 205,000 48,000 2002
Intercasino
#9 Popular UK Casino 4,500,000 180,000 40,000 1996
Mansion Casino
#10 Popular UK Casino 1,058,000 160,000 23,000 2004
Betfred Casino
#11 Popular UK Casino 1,210,000 130,000 15,000 2004
Cirrus UK
#12 Popular UK Casino 650,000 75,000 7250 2001
Paddy Power
#13 Popular UK Casino 430,000 30,000 6,000 2009
Brands with an Asian presence too are undoubtedly heaving in terms of player numbers. The last time I looked in to this in terms of Playtech operators, I'm pretty sure a Japanese brand topped all the ones you'd have heard of (I can't even recall the name).